toolchain/gcc: switch to version 12 by default

toolchain/gcc: switch to version 12 by default

This was build tested with all core packages on all targets
successfully.

Most packages from the feed are also building fine.

This was run tested on the following systems:
* lantiq/xrx200 musl
* pistachio/generic musl
* sunxi/cortexa53 musl
* x86/64 musl
* x86/64 glibc
* armvirt/64 musl

The size of the images stays more or less the same for MIPS BE and
aarch64. I haven't tested other architectures.

With GCC 11 I got these sizes for lantiq/xrx200:
7,219,848 openwrt-lantiq-xrx200-tplink_tdw8970-initramfs-kernel.bin
7,472,208 openwrt-lantiq-xrx200-tplink_tdw8970-squashfs-sysupgrade.bin

With GCC 12 I got these sizes for lantiq/xrx200:
7,217,355 openwrt-lantiq-xrx200-tplink_tdw8970-initramfs-kernel.bin
7,406,674 openwrt-lantiq-xrx200-tplink_tdw8970-squashfs-sysupgrade.bin

The sysupgrade image is probably padded. The initramfs image is 0.03% smaller.

With GCC 11 I got these sizes for armvirt/64:
 4,143,943 openwrt-armvirt-64-default-rootfs.tar.gz
10,887,176 openwrt-armvirt-64-Image
24,911,880 openwrt-armvirt-64-Image-initramfs
 4,141,572 openwrt-armvirt-64-rootfs.cpio.gz
 4,255,854 openwrt-armvirt-64-rootfs-ext4.img.gz
 3,391,178 openwrt-armvirt-64-rootfs-squashfs.img.gz

With GCC 12 I got these sizes for armvirt/64:
 4,142,778 openwrt-armvirt-64-default-rootfs.tar.gz
10,887,176 openwrt-armvirt-64-Image
24,911,880 openwrt-armvirt-64-Image-initramfs
 4,138,105 openwrt-armvirt-64-rootfs.cpio.gz
 4,255,463 openwrt-armvirt-64-rootfs-ext4.img.gz
 3,390,390 openwrt-armvirt-64-rootfs-squashfs.img.gz

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>